The coal industry reduces the moisture content of coal prior to shipping the coal to utilities and industrial coal burning customers to increase the heating value of the coal by reducing the coal's weight per unit of energy. The price of coal sold to utilities depends upon the heating value of the coal at the mine. In addition to bringing a higher unit price for coal, reducing the moisture content of coal increases the efficiency of the power plant, decreases transportation costs, decreases ash disposal requirements, and decreases power plant emissions.
An important consideration in reducing the moisture content of coal is the manner in which moisture is contained in coal particles. Coal particles typically have both a surface and an inherent moisture content. The inherent moisture content represents water retained in the cells and capillaries distributed .throughout the coal particle matrix. As used herein, "cells" refers to discrete void spaces in the coal particle matrix and "capillaries" refers to vein-like fissures connecting the cells.
The surface and inherent moisture contents of coal may be reduced in a variety of apparatuses including travelling bed driers, autoclaves, cascading bed driers, and fluidized bed driers. As used herein, "fluidized bed drier" refers to an apparatus which reduces the moisture content of a particle by passing a gas, including hot products of combustion and steam, through a bed of particles. The bed of particles in the fluidized bed drier is known as a fluidized bed and the gas passing through the fluidized bed is known as the fluidizing gas. The primary objectives of conventional coal drying operations are to obtain the lowest possible moisture content in the coal particles for a variety of end uses such as combustion, coking, and gasification, and/or obtain a high density and to cause a reduced likelihood for spontaneous combustion of the coal particles by collapsing as many cells and capillaries as possible. Cells and capillaries are believed to collapse as water contained in the cells and capillaries is removed.
Conventional coal drying operations have a number of problems. First, the drying operations tend to be quite complex which translates into high capital and operating costs. For example, many operations employ multiple heat exchangers, condensers, cyclones, and so forth in addition to complex and expensive fire prevention equipment. Second, many operations operate at superatmospheric pressure to collapse cells and capillaries which requires more expensive equipment to resist the increased operating pressures. Third, many operations require excessive amounts of heat to dry the coal. The high amounts of heat result both from the need to heat the system to high drying temperatures and the inefficiencies caused by high heat losses and poor heat exchange rates. Finally, conventional coal drying processes generate excessive amounts of coal fines. Coal fines are created during the drying of the coal as water is removed. The removal of water from the coal weakens the coal structure causing attrition in particle size.
The suppression of the coal fines generated during the drying of coal is a major problem in the coal and utility industries. Releases of coal fines during transportation, processing, and handling of coal cause health and safety problems and pollution. Such releases also increase operating costs for the coal and utility companies through unrecoverable coal losses and complications in coal handling and storage.
Coal fines may be compressed into agglomerates, such as briquettes. As used herein, "agglomerate" refers to any consolidation of particles to form a consolidated mass and "briquette," as used herein, refers to an agglomerate produced by compressing particles under externally applied pressure. There are numerous methods to compress coal particles into briquettes, including but not limited to extruding, ringrolling, roll pressing, and die pressing.
Coal may be compressed into briquettes with or without a binder. As used herein, "binder" refers to an additive to produce or promote cohesion in loosely assembled substances. Binders are more expensive than the coal itself and substantially increase costs associated with briquetting.
Binderless briquetting may be done by two methods. First, a fraction of the coal particles may be heated to their softening point and then compressed to bind the coal particles with naturally occurring tar pitch. This approach is extremely expensive as it requires the coal to be heated to temperatures in excess of 300.degree. C. Second, briquetting may combine the adhesive effects of high compaction pressures and coal moisture to cause particle consolidation. This type of binderless briquetting demands careful preparation of the coal particles to produce strong briquettes, thereby substantially increasing coal costs. The preparation includes crushing the coal particles to a fine size, conditioning the coal particles for briquetting and compressing the coal particles under extreme pressure. Coal particle conditioning is a significant problem in conventional briquetting operations as coal particles dried in conventional drying operations do not make strong briquettes without the addition of substantial amounts of binder. In both binder and binderless briquetting there is the additional problem that coal briquettes often swell and crack during cooling.